Control for bfd return path

ABSTRACT

A method is implemented by a network device to establish a bidirectional forwarding detection (BFD) session with a defined return path to enable detection of data plane failures between an active node and a passive node in a network where a forward path and a reverse path between the active node and the passive node are not co-routed. The method includes receiving a label switched path (LSP) ping including a BFD discriminator type length value (TLV) of the active node and a return path TLV describing a return path to the active node. The BFD discriminator of the active node and a BFD discriminator of the passive node are associated with the BFD session. The return path is associated with the BFD session between the active node and the passive node, and BFD control packets are sent to the active node using the return path to detect a failure on the return path.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.14/512,259 filed Oct. 10, 2014, which claims the benefit of U.S.Provisional Application No. 62/019,244, filed Jun. 30, 2014, which isincorporated herein by reference.

FIELD

Embodiments of the invention relate to detection of data plane failures.Specifically, the embodiments relate to a method for detecting dataplane failures when explicit paths and best route paths utilized in theforward and reverse directions between two nodes in a network are notco-routed.

BACKGROUND

Bidirectional forwarding detection (BFD) is a network protocol used todetect faults between two nodes in a network connected by a link or pathacross the network. The Internet Engineering Task Force (IETF) Requestfor Comments (RFC) 5880 sets forth that the base protocol for“Bidirectional Forwarding Detection (BFD)” may apply to any network,including to Internet Protocol (IP) networks. Predominantly, pathselection between nodes in IP networks is controlled through dynamicrouting protocols and selection of the best available route to a givennetwork node. Routing protocols and rules of selecting the best routeare designed to ensure that an IP packet follows the best route (e.g.,the route with the lowest cost measure) from one node to another nodetowards a destination without causing the IP packet to loop in thenetwork.

The BFD protocol uses control packets to detect failures in the network.These control packets are sent between the end nodes of a BFD session,where a failure of N consecutive control packets to arrive at theopposite end indicates a failure in the link or path, where N is definedas the Detect Multiplier in RFC 5880 and may be set to 1, but more oftenhas a value of 3. BFD control packets, like other IP data packets, willfollow the best route to the destination node and thus ensure an in-bandrequirement that is set forth toward Operation, Administration, andMaintenance (OAM) mechanisms. Because IP routing is usually symmetrical(i.e., the best routes between two IP end points traverse the samenodes), BFD control packets in the forward and reverse directions of aBFD session between two nodes usually traverse the same nodes. As aresult, any detected unidirectional failure in the link or path can beconsidered as a bidirectional defect and can be acted upon accordingly.However, the best route model does not allow control of trafficdistribution beyond the best route paths.

An alternative to the best route paradigm is a method wherein an entirepath is predetermined, for example, either by a controller or at theingress node. This can be done either by directing packets into anexplicitly-routed tunnel or by explicitly specifying all intermediatenodes for each packet. In general, such paths need to be viewed asunidirectional. Because BFD is predominantly used in its asynchronousmode, the far-end peer node is likely to select the best available routewhen sending its BFD control packets. As a result, BFD control packetsmay not cross the same set of nodes in the forward and reversedirections. Thus, a unidirectional failure cannot be interpreted as anindication of a bidirectional defect, and in certain scenarios, a defectwould not be detected for an extended period of time.

SUMMARY

A method is implemented by a network device to establish a bidirectionalforwarding detection (BFD) session with a defined return path to enabledetection of data plane failures between an active node and a passivenode in a network where a forward path and a reverse path between theactive node and the passive node are not co-routed. The method includesreceiving a label switched path (LSP) ping including a BFD discriminatortype length value (TLV) of the active node and a return path TLVdescribing a return path to the active node. A BFD discriminator of theactive node and a BFD discriminator for the passive node are associatedwith the BFD session. The return path is associated with the BFD sessionbetween the active node and the passive node, and BFD control packetsare sent to the active node using the return path to detect a failure onthe return path.

A network device establishes a bidirectional forwarding detection (BFD)session with a defined return path to enable detection of data planefailures between an active node and a passive node in a network where aforward path and a reverse path between the active node and the passivenode are not co-routed. The network device includes a non-transitorymachine-readable storage medium configured to store a BFD module and areturn path module and a network processor. The network processor iscommunicatively coupled to the non-transitory machine-readable storagemedium. The network processor is configured to execute the BFD moduleand the return path module. The BFD module is configured to receive anLSP ping including a BFD discriminator TLV of the active node and areturn path TLV describing a return path to the active node, and toassociate a BFD discriminator of the active node and a BFD discriminatorfor the passive node with the BFD session. The return path module isconfigured to associate the return path with a BFD session between theactive node and the passive node, and to send BFD control packets to theactive node using the return path to detect a failure on the returnpath.

A computing device executes a plurality of virtual machines forimplementing network function virtualization (NFV), wherein a virtualmachine from the plurality of virtual machines is configured to executea method to establish a bidirectional forwarding detection (BFD) sessionwith a defined return path to enable detection of data plane failuresbetween an active node and a passive node in a network where a forwardpath and a reverse path between the active node and the passive node arenot co-routed. The computing device includes a non-transitorymachine-readable storage medium configured to store a BFD module and areturn path module. The computing device also includes a processorcommunicatively coupled to the non-transitory machine-readable storagemedium. The processor is configured to execute the virtual machine thatexecutes the BFD module and the return path module. The BFD module isconfigured to receive an LSP ping including a BFD discriminator TLV ofthe active node and a return path TLV describing a return path to theactive node, and to associate a BFD discriminator of the active node anda BFD discriminator of the passive node with the BFD session. The returnpath module is configured to associate the return path with a BFDsession between the active node and the passive node, and to send BFDcontrol packets to the active node using the return path to detect afailure on the return path.

A control plane device is configured to implement at least onecentralized control plane for a software defined network (SDN). Thecentralized control plane is configured to execute a method to establisha bidirectional forwarding detection (BFD) session with a defined returnpath to enable detection of data plane failures between an active nodeand a passive node in a network where a forward path and a reverse pathbetween the active node and the passive node are not co-routed. Thecontrol plane device includes a non-transitory machine-readable storagemedium configured to store a BFD module and a return path module and aprocessor communicatively coupled to the non-transitory machine-readablestorage medium. The processor is configured to execute the BFD moduleand the return path module. The BFD module is configured to determine aBFD discriminator of the active node and a return path to the activenode, and to associate the BFD discriminator of the active node and aBFD discriminator of the passive node with the BFD session. The returnpath module is configured to associate the return path with a BFDsession between the active node and the passive node, and to configurethe passive node to send BFD control packets to the active node usingthe return path to detect a failure on the return path.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. In the drawings:

FIG. 1 is a diagram of one embodiment of an example network in whichbidirectional forwarding detection (BFD) is implemented.

FIG. 2A is a flowchart of one embodiment of a process for establishing aBFD session by a near-end node.

FIG. 2B is a flowchart of one embodiment of a process for establishing aBFD session by a far-end node.

FIG. 3 is a diagram of one embodiment a network device (ND) implementingan asynchronous BFD session with support for explicit return paths.

FIG. 4A illustrates connectivity between network devices (NDs) within anexemplary network, as well as three exemplary implementations of theNDs, according to some embodiments of the invention.

FIG. 4B illustrates an exemplary way to implement the special-purposenetwork device 402 according to some embodiments of the invention.

FIG. 4C illustrates various exemplary ways in which virtual networkelements (VNEs) may be coupled according to some embodiments of theinvention.

FIG. 4D illustrates a network with a single network element (NE) on eachof the NDs of FIG. 4A.

FIG. 4E illustrates an example where each of the NDs implements a singleNE (see FIG. 4D), but the centralized control plane has abstractedmultiple ones of the NEs in different NDs into a single NE in one of thevirtual network(s) of FIG. 4D, according to some embodiments of theinvention.

FIG. 4F illustrates a case where multiple VNEs are implemented ondifferent NDs and are coupled to each other, and where the centralizedcontrol plane has abstracted these multiple VNEs such that they appearas a single VNE within one of the virtual networks of FIG. 4D, accordingto some embodiments of the invention.

FIG. 5 illustrates a general purpose control plane device includinghardware comprising a set of one or more processor(s) (which are oftenCommercial off-the-shelf (COTS) processors) and network interfacecontroller(s) (NICs; also known as network interface cards) (whichinclude physical NIs), as well as non-transitory machine readablestorage media having stored therein centralized control plane (CCP)software), according to some embodiments of the invention.

DETAILED DESCRIPTION

The following description describes methods and apparatus forestablishing an asynchronous BFD session with support for defining anexplicit return path that can be co-routed or not co-routed with aforward path. In the following description, numerous specific detailssuch as logic implementations, opcodes, means to specify operands,resource partitioning/sharing/duplication implementations, types andinterrelationships of system components, and logicpartitioning/integration choices are set forth in order to provide amore thorough understanding of the present invention. It will beappreciated, however, by one skilled in the art that the invention maybe practiced without such specific details. In other instances, controlstructures, gate level circuits and full software instruction sequenceshave not been shown in detail in order not to obscure the invention.Those of ordinary skill in the art, with the included descriptions, willbe able to implement appropriate functionality without undueexperimentation.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to affect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

Bracketed text and blocks with dashed borders (e.g., large dashes, smalldashes, dot-dash, and dots) may be used herein to illustrate optionaloperations that add additional features to embodiments of the invention.However, such notation should not be taken to mean that these are theonly options or optional operations, and/or that blocks with solidborders are not optional in certain embodiments of the invention.

In the following description and claims, the terms “coupled” and“connected,” along with their derivatives, may be used. It should beunderstood that these terms are not intended as synonyms for each other.“Coupled” is used to indicate that two or more elements, which may ormay not be in direct physical or electrical contact with each other,co-operate or interact with each other. “Connected” is used to indicatethe establishment of communication between two or more elements that arecoupled with each other.

The operations in the flow diagrams will be described with reference tothe exemplary embodiments of the other figures. However, it should beunderstood that the operations of the flow diagrams can be performed byembodiments of the invention other than those discussed with referenceto the other figures, and the embodiments of the invention discussedwith reference to these other figures can perform operations differentthan those discussed with reference to the flow diagrams.

Overview

The IETF RFC 5880, the IETF RFC 5881 entitled “Bidirectional ForwardingDetection (BFD) for IPv4 and IPv6 (Single Hop),” and the IETF RFC 5883entitled “Bidirectional Forwarding Detection (BFD) for Multihop Paths”established the BFD protocol for Internet Protocol (IP) networks. TheIETF RFC 5884 entitled “Bidirectional Forwarding Detection (BFD) forMPLS Label Switched Paths (LSPs)” describes a procedure for using BFD inasynchronous mode over IP/Multiprotocol Label Switching (MPLS) LSPs.These standards implicitly presume that the best route (e.g., the routewith the lowest cost measure) between a near-end node establishing a BFDsession and a far-end peer node of the BFD session will be used,regardless of the actual route used by that far-end peer node to sendits BFD control packets to the near-end node. The term near-end nodeindicates a node that initiates a BFD session with a target node. Theterm far-end node indicates the target node of a BFD session. These canbe any two nodes in a network that support the BFD related standards.

BFD has two operational modes, (1) demand mode and (2) asynchronousmode. In demand mode, no hello packets are automatically exchanged afterthe session is established; rather, the two endpoints can exchange hellopackets as needed. Asynchronous mode provides the scheduled or periodicexchange of hello packets or similar BFD control packets between the twoend nodes. When these BFD control packets fail to arrive at thedestination node then a failure is assumed to have occurred along thepath. The embodiments described herein contemplate a modification to theasynchronous mode of operation.

As discussed above, BFD is expected to monitor bidirectional co-routedpaths (i.e., the forward direction path from the near-end node to thefar-end node, and the return direction path from the far-end node to thenear-end node). In most IP and IP/MPLS networks, the best route betweentwo IP nodes is likely to be co-routed in a stable network environment,and thus, that implicit BFD requirement is fulfilled. However, when theforward direction path of a BFD session is explicitly routed (e.g., notusing the best route) while the return direction path of the BFD sessionuses the best route, several scenarios present a problem:

-   -   a. A failure detected on the return direction path cannot be        interpreted as a bidirectional failure, because the forward and        return direction paths are not co-routed. If it is incorrectly        interpreted as a bidirectional failure, unnecessary protection        switchover of the forward direction path may occur.    -   b. If the return direction path is in the down state, the        near-end peer node would not receive an indication of a forward        direction path failure from its far-end peer node.

FIG. 1 illustrates nodes and links of an example network wherein theBFD-monitored paths in the forward and return directions between peernodes A and E are not co-routed. This example network is used to showthe problems in BFD monitoring where the forward direction path and thereturn direction path are not co-routed. In the example BFD session ofFIG. 1, peer node A is the near-end peer node, and peer node E is thefar-end peer node. The path in the forward direction between peer nodesA and E is explicitly routed as A-B-C-D-E. In contrast, the path in thereturn direction between peer nodes E and A is the best route E-G-F-A.Thus, the BFD-monitored paths in the forward and return directions arenot co-routed at any intermediate node or link. If a failure on the pathin the return direction (e.g., a failure in the link between nodes F andG) is detected by peer node A, that failure should not be interpreted asa failure on the path in the forward direction, on the explicitly-routedA-B-C-D-E path. Moreover, if peer node E detects a failure on the pathin the forward direction (e.g., a failure in the link between nodes Cand D on the explicitly-routed A-B-C-D-E path), then peer node A shouldnot receive an indication of this forward direction failure from peernode E, even for example, if the return direction path is not in thedown state, as the peer node E can continue to function on its returnpath.

The embodiments of the invention provide a system and method to overcomethese limitations of the existing standards, such that the method to useBFD in MPLS networks can properly handle non-co-routed forward directionpath and return direction path. In the embodiments, a session initiatingnear-end peer node uses LSP Ping with a BFD DiscriminatorType-Length-Value (TLV) so that the far-end peer node can properlyassociate the BFD session with the sending near-end peer node. Ineffect, LSP Ping bootstraps a BFD session between two peer nodes, e.g.,Label Switch Routers (LSRs). The embodiments further introduce amechanism for the near-end peer node to instruct the far-end peer nodeto use a particular path or an explicit MPLS label stack to send thefar-end peer node's BFD control packets in the return direction of theBFD session that is being bootstrapped.

The embodiments further overcome the limitations of the prior art suchthat a near-end peer node (e.g., an LSR) that is to monitor anexplicitly-routed unidirectional LSP uses LSP Ping to bootstrap the BFDsession by informing the far-end peer node (e.g., an LSR) of thenear-end peer node's BFD Discriminator (i.e., using the BFDDiscriminator TLV). The near-end peer node includes an LSP identifier(ID) or lists an explicit MPLS label stack in a BFD Return Path TLV(referred to herein simply as the return path TLV) to be used by thefar-end peer node to send the far-end peer node's BFD control packets inthe return direction.

When the far-end peer node receives the LSP Ping, the far-end peer nodelocates the BFD Discriminator TLV from the LSP Ping and associates theLSP with the value of the near-end peer node's BFD Discriminator. Thefar-end peer node then locates the BFD Return Path TLV with the LSP IDor the explicit MPLS label stack from the LSP Ping. The far-end peernode verifies availability of the specified path (i.e. the pathindicated by the LSP ID or the explicit MPLS label stack) and, if theavailability is verified, associates this specified path with the BFDsession. The far-end peer node then begins to send its BFD controlpackets with the My Discriminator value set to the far-end peer node'sBFD Discriminator, and the Your Discriminator value set to the valuereceived in the BFD Discriminator TLV (i.e., the near-end peer node'sBFD Discriminator) over the path specified in the BFD Return Path TLV.Alternatively, if the specified path is not available, the far-end peernode notifies the near-end peer node of the error using an LSP PingReply and may use the best route to send BFD control packets in thereturn direction.

In other embodiments, other mechanisms may be used to instruct thefar-end peer node (e.g., an LSR) to use a particular return path to sendBFD control packets in the return direction. For example, the path couldbe collected as the packet flies by, centralized control (e.g., via anOperations Support System (OSS) or Software-Defined networking (SDN))could be used to retrieve information about, or an indication of, theparticular path to use in the reverse direction, the particular pathcould be encoded in an identifier, or similar techniques can beimplemented.

The embodiments provide significant benefits over the prior art BFDsystems. For example, directing the far-end peer node of the BFD sessionto use a particular path for sending the far-end peer node's BFD controlpackets in the return direction enables those BFD control packets totraverse the same nodes and links as traversed in the forward direction,even when the BFD-monitored path in the forward direction is explicitlyrouted. Moreover, because the BFD session is maintained over abidirectional co-routed path when using the process described herein,detection of a failure in one direction can be reliably interpreted andtreated as an indication of a bidirectional defect in the path.

FIG. 2A is a flowchart of one embodiment the process for establishing aBFD session by the near-end node. The near-end node can be any node inany type of network. A node, as referred to herein, is a computingdevice or networking device that can communicate over a connectednetwork with other nodes. The BFD session can be initiated as directedby a user or based on a local policy of the implementing device. Thenear-end node can also be referred to herein as an ‘active’ node todistinguish it from the far-end node or ‘passive’ node. The active andpassive nodes are peer nodes in the network with a set of defined pathssuch as LSPs between them. A ‘set,’ as used herein refers to anypositive whole number of items including at least one item. In thiscontext, the active node and passive node are assumed to have at leasttwo paths between them to enable the possibility of a forward path and areturn path between the active node and passive node that are notco-located such that a BFD session can be established with differingforward and return paths.

The process begins with initiating the BFD session by the active nodesending an LSP ping to the passive node (Block 201). The LSP ping isutilized to bootstrap the BFD session, since BFD does not have defined aremote initiation process. The LSP ping is a type of message and, asused herein, refers to a modified form of the message that includes adiscriminator TLV and a return path TLV. The BFD discriminator TLVincludes a BFD discriminator that is a value utilized in the network todistinguish between nodes in the network that are communicating with thereceiving node. The BFD discriminator is locally unique to the receivingnode, but may not be globally unique for the network.

The return path TLV can identify a path between the active node andpassive node by including an LSP identifier or an MPLS stack, which is aset of MPLS labels for traversing LSRs between the active node andpassive node. In further embodiments, any system or format foridentifying a path between the active node and passive node can beutilized such that it can be stored in a return path TLV. The LSP pingcontaining the BFD discriminator TLV and return path TLV causes thepassive node to establish the BFD session.

The passive node establishes the BFD session using the BFD discriminatorprovided by the BFD discriminator TLV and the return path identified bythe return path TLV to send BFD control packets to the active node alongthe return path, such that the active node receives the BFD controlpackets (Block 203). This assumes that no error occurred at the passivenode when it processed the LSP ping. If an error did occur, then an LSPping reply would be sent by the passive node and received by the activenode (Block 203). To properly establish the BFD session, the BFD controlpackets sent by the passive node must include the BFD discriminator ofthe active node. The active node begins receiving the BFD controlpackets over the return path and can thereafter detect a distinctfailure in the return path or forward path when the BFD control packetsare not received at either the active node or passive node. Operation ofthe BFD session can continue indefinitely until a failure occurs or theBFD session is terminated by either the active node or passive node.

FIG. 2B is a flowchart of one embodiment of the process for establishingthe BFD session as implemented by the far-end or passive node inresponse to receiving an LSP ping. The passive node can be any node inthe network other than the active node and is assumed to have at leasttwo paths to the active node to enable the possibility of a forward pathand a return path between the active node and passive node that are notco-located such that a BFD session can be established with differingforward and return paths. The passive node is configured to support BFDand bootstrapping of BFD sessions using LSP ping messages as set forthherein. The process of establishing the BFD session is initiated at thepassive node when an LSP ping is received (Block 251). The LSP ping canbe received over any port of the passive node and from any other node inthe network. In the example, the LSP ping is sent by the active node.The LSP ping is modified to include at least a BFD discriminator TLV anda return path TLV. The BFD discriminator TLV includes a BFDdiscriminator, which is a value that identifies the sending node and isunique at least relative to the passive node. The return path TLVincludes data describing a path between the active node and the passivenode. The path can be identified using an LSP identifier, a MPLS stack(i.e., a set of labels that identify how a packet is to be forwardedacross a set of nodes) or similar format or mechanism for identifying apath in the network. Using an explicitly defined return path can beutilized for any purpose, for example, to monitor additional paths, tomatch how other data flows are handled between the active and passivenodes, or for similar reasons.

After receipt of the LSP ping, the passive node can access the contentof the LSP ping message to retrieve the BFD discriminator for the activenode from the BFD discriminator TLV of the message (Block 253). Theinformation in the LSP ping is also verified by the passive nodeincluding the Target Forward Equivalency Class (FEC) Stack TLV and theencapsulation of the LSP ping. If this information is validated and if aBFD discriminator was located in the BFD discriminator TLV, then the BFDdiscriminator of the active node can then be associated with the BFDsession being established (Block 255). In addition, a BFD discriminatorthat is locally unique for the passive node is associated with the BFDsession. BFD control packets sent to the active node will include theBFD discriminator of the active node and a BFD discriminator of thepassive node. The passive node can also access and retrieve the returnpath TLV (Block 257). The description of the return path can then beanalyzed and associated with the BFD session.

The analysis of the path described by the return path TLV can includeverifying the availability of the return path (Block 259). Theverification can use any process or mechanism to check the receivedreturn path against the network topology available to the passive node.Path availability can be determined on whether the active node isreachable according to the known topology of the passive node along thespecified return path or based on the reachability of intermediate nodessuch as the first hop according to the return path. If the path is notavailable, then the active node is notified of the error by generatingand sending the error as part of an LSP ping reply or using a similarmechanism (Block 261). The passive node then uses the best availableroute to send the BFD control packets to the active node instead of thereturn path described by the received return path TLV (Block 267). Thebest available route can be determined by using any routing algorithm,for example, the general routing algorithm utilized by the network todetermine paths using the shared network topology.

If the received return path is available, then the passive node utilizesthe specified path for sending the BFD control packets to the activenode. The return path is associated with the BFD session between theactive node and the passive node (Block 263). The sending of the BFDcontrol packets to the active node using the return path can thencontinue for any period of time or during the duration of the BFDsession (Block 265). The exchange of the BFD control packets enables theseparate detection of the failure of the forward path and the returnpath between the active node and the passive node.

FIG. 3 illustrates an example of a network device 301 to implementreturn path control functions for BFD.

A network device (ND) is an electronic device that communicativelyinterconnects other electronic devices on the network (e.g., othernetwork devices, end-user devices). Some network devices are “multipleservices network devices” that provide support for multiple networkingfunctions (e.g., routing, bridging, switching, Layer 2 aggregation,session border control, Quality of Service, and/or subscribermanagement), and/or provide support for multiple application services(e.g., data, voice, and video).

In one embodiment, the process of FIG. 2A and/or FIG. 2B is implementedby a router 301 or network device or similar computing device. Therouter 301 can have any structure that enables it to receive datatraffic and forward it toward its destination. The router 301 caninclude a network processor 303 or a set of network processors thatexecute the functions of the router 301. The router 301 or networkelement can execute BFD including the explicit return path definitionfunctionality via a network processor 303 or other components of therouter 301. The network processor 303 can implement the BFD and returnpath definition functions stored as a BFD module 307 and the return pathmodule 351, wherein the functions include the BFD session initiation,bootstrapping, return path definition and verification described hereinabove. The return path module 351 may be part of the BFD module 307, asillustrated in FIG. 3, or separate from the BFD module 307. The networkprocessor 303 can also service the routing information base 305A andsimilar functions related to data traffic forwarding and networktopology maintenance.

The BFD and return path definition functions can be implemented asmodules in any combination of software, including firmware, and hardwarewithin the router. The functions of the BFD and return path definitionprocesses that are executed and implemented by the router 301 includethose described further herein above including the bootstrapping usingthe LSP ping to provide return path information as part of theestablishment of a BFD session between two nodes.

In one embodiment, the router 301 can include a set of line cards 317that process and forward the incoming data traffic toward the respectivedestination nodes by identifying the destination and forwarding the datatraffic to the appropriate line card 317 having an egress port thatleads to or toward the destination via a next hop. These line cards 317can also implement the forwarding information base 305B, or a relevantsubset thereof. The line cards 317 can also implement or facilitate theBFD and return path definition functions described herein above. Forexample, the line cards 317 can implement LSP ping and LSP ping replyfunctions and similar functions. The line cards 317 are in communicationwith one another via a switch fabric 311 and communicate with othernodes over attached networks 321 using Ethernet, fiber optic or similarcommunication links and media.

The operations of the flow diagrams have been described with referenceto the exemplary embodiment of the block diagrams. However, it should beunderstood that the operations of the flowcharts can be performed byembodiments of the invention other than those discussed, and theembodiments discussed with reference to block diagrams can performoperations different than those discussed with reference to theflowcharts. While the flowcharts show a particular order of operationsperformed by certain embodiments, it should be understood that suchorder is exemplary (e.g., alternative embodiments may perform theoperations in a different order, combine certain operations, overlapcertain operations, etc.).

As described herein, operations performed by the router may refer tospecific configurations of hardware such as application specificintegrated circuits (ASICs) configured to perform certain operations orhaving a predetermined functionality, or software instructions stored inmemory embodied in a non-transitory computer readable storage medium.Thus, the techniques shown in the figures can be implemented using codeand data stored and executed on one or more electronic devices (e.g., anend station, a network element). Such electronic devices store andcommunicate (internally and/or with other electronic devices over anetwork) code and data using computer-readable media, such asnon-transitory computer-readable storage media (e.g., magnetic disks;optical disks; random access memory; read only memory; flash memorydevices; phase-change memory) and transitory computer-readablecommunication media (e.g., electrical, optical, acoustical or other formof propagated signals—such as carrier waves, infrared signals, digitalsignals). In addition, such electronic devices typically include a setof one or more processors coupled to one or more other components, suchas one or more storage devices (non-transitory machine-readable storagemedia), user input/output devices (e.g., a keyboard, a touchscreen,and/or a display), and network connections. The coupling of the set ofprocessors and other components is typically through one or more bussesand bridges (also termed as bus controllers). Thus, the storage deviceof a given electronic device typically stores code and/or data forexecution on the set of one or more processors of that electronicdevice. One or more parts of an embodiment of the invention may beimplemented using different combinations of software, firmware, and/orhardware.

An electronic device stores and transmits (internally and/or with otherelectronic devices over a network) code (which is composed of softwareinstructions and which is sometimes referred to as computer program codeor a computer program) and/or data using machine-readable media (alsocalled computer-readable media), such as machine-readable storage media(e.g., magnetic disks, optical disks, read only memory (ROM), flashmemory devices, phase change memory) and machine-readable transmissionmedia (also called a carrier) (e.g., electrical, optical, radio,acoustical or other form of propagated signals—such as carrier waves,infrared signals). Thus, an electronic device (e.g., a computer)includes hardware and software, such as a set of one or more processorscoupled to one or more machine-readable storage media to store code forexecution on the set of processors and/or to store data. For instance,an electronic device may include non-volatile memory containing the codesince the non-volatile memory can persist code/data even when theelectronic device is turned off (when power is removed), and while theelectronic device is turned on that part of the code that is to beexecuted by the processor(s) of that electronic device is typicallycopied from the slower non-volatile memory into volatile memory (e.g.,dynamic random access memory (DRAM), static random access memory (SRAM))of that electronic device. Typical electronic devices also include a setor one or more physical network interface(s) to establish networkconnections (to transmit and/or receive code and/or data usingpropagating signals) with other electronic devices. One or more parts ofan embodiment of the invention may be implemented using differentcombinations of software, firmware, and/or hardware.

FIG. 4 illustrates connectivity between network devices (NDs) within anexemplary network, as well as three exemplary implementations of theNDs, according to some embodiments of the invention. FIG. 4A shows NDs400A-H, and their connectivity by way of lines between A-B, B-C, C-D,D-E, E-F, F-G, and A-G, as well as between H and each of A, C, D, and G.These NDs are physical devices, and the connectivity between these NDscan be wireless or wired (often referred to as a link). An additionalline extending from NDs 400A, E, and F illustrates that these NDs act asingress and egress points for the network (and thus, these NDs aresometimes referred to as edge NDs; while the other NDs may be calledcore NDs).

Two of the exemplary ND implementations in FIG. 4A are: 1) aspecial-purpose network device 402 that uses custom application-specificintegrated-circuits (ASICs) and a proprietary operating system (OS); and2) a general purpose network device 404 that uses common off-the-shelf(COTS) processors and a standard OS.

The special-purpose network device 402 includes networking hardware 410comprising compute resource(s) 412 (which typically include a set of oneor more processors), forwarding resource(s) 414 (which typically includeone or more ASICs and/or network processors), and physical networkinterfaces (NIs) 416 (sometimes called physical ports), as well asnon-transitory machine readable storage media 418 having stored thereinnetworking software 420. A physical NI is hardware in a ND through whicha network connection (e.g., wirelessly through a wireless networkinterface controller (WNIC) or through plugging in a cable to a physicalport connected to a network interface controller (NIC)) is made, such asthose shown by the connectivity between NDs 1200A-H. During operation,the networking software 420 may be executed by the networking hardware410 to instantiate a set of one or more networking software instance(s)422. Each of the networking software instance(s) 422, and that part ofthe networking hardware 410 that executes that network software instance(be it hardware dedicated to that networking software instance and/ortime slices of hardware temporally shared by that networking softwareinstance with others of the networking software instance(s) 422), form aseparate virtual network element 430A-R. Each of the virtual networkelement(s) (VNEs) 430A-R includes a control communication andconfiguration module 432A-R (sometimes referred to as a local controlmodule or control communication module) and forwarding table(s) 434A-R,such that a given virtual network element (e.g., 430A) includes thecontrol communication and configuration module (e.g., 432A), a set ofone or more forwarding table(s) (e.g., 434A), and that portion of thenetworking hardware 410 that executes the virtual network element (e.g.,430A). In some embodiments, the control communication and configurationmodule 432A encompasses the BFD module 433A that establishes BFDsessions over connected links with remote nodes in the network. A returnpath module 492A can similarly manage the handling of the explicitdefinition and exchange of return paths where they differ from theforward paths of the BFD sessions.

A network interface (NI) may be physical or virtual; and in the contextof IP, an interface address is an IP address assigned to a NI, be it aphysical NI or virtual NI. A virtual NI may be associated with aphysical NI, with another virtual interface, or stand on its own (e.g.,a loopback interface, a point-to-point protocol interface). A NI(physical or virtual) may be numbered (a NI with an IP address) orunnumbered (a NI without an IP address). A loopback interface (and itsloopback address) is a specific type of virtual NI (and IP address) of aNE/VNE (physical or virtual) often used for management purposes; wheresuch an IP address is referred to as the nodal loopback address. The IPaddress(es) assigned to the NI(s) of a ND are referred to as IPaddresses of that ND; at a more granular level, the IP address(es)assigned to NI(s) assigned to a NE/VNE implemented on a ND can bereferred to as IP addresses of that NE/VNE.

The special-purpose network device 402 is often physically and/orlogically considered to include: 1) a ND control plane 424 (sometimesreferred to as a control plane) comprising the compute resource(s) 412that execute the control communication and configuration module(s)432A-R; and 2) a ND forwarding plane 426 (sometimes referred to as aforwarding plane, a data plane, or a media plane) comprising theforwarding resource(s) 414 that utilize the forwarding table(s) 434A-Rand the physical NIs 416. By way of example, where the ND is a router(or is implementing routing functionality), the ND control plane 424(the compute resource(s) 412 executing the control communication andconfiguration module(s) 432A-R) is typically responsible forparticipating in controlling how data (e.g., packets) is to be routed(e.g., the next hop for the data and the outgoing physical NI for thatdata) and storing that routing information in the forwarding table(s)434A-R, and the ND forwarding plane 426 is responsible for receivingthat data on the physical NIs 416 and forwarding that data out theappropriate ones of the physical NIs 416 based on the forwardingtable(s) 434A-R.

FIG. 4B illustrates an exemplary way to implement the special-purposenetwork device 402 according to some embodiments of the invention. FIG.4B shows a special-purpose network device including cards 438 (typicallyhot pluggable). While in some embodiments the cards 438 are of two types(one or more that operate as the ND forwarding plane 426 (sometimescalled line cards), and one or more that operate to implement the NDcontrol plane 424 (sometimes called control cards)), alternativeembodiments may combine functionality onto a single card and/or includeadditional card types (e.g., one additional type of card is called aservice card, resource card, or multi-application card). A service cardcan provide specialized processing (e.g., Layer 4 to Layer 7 services(e.g., firewall, Internet Protocol Security (IPsec) (RFC 4301 and 4309),Secure Sockets Layer (SSL)/Transport Layer Security (TLS), IntrusionDetection System (IDS), peer-to-peer (P2P), Voice over IP (VoIP) SessionBorder Controller, Mobile Wireless Gateways (Gateway General PacketRadio Service (GPRS) Support Node (GGSN), Evolved Packet Core (EPC)Gateway)). By way of example, a service card may be used to terminateIPsec tunnels and execute the attendant authentication and encryptionalgorithms. These cards are coupled together through one or moreinterconnect mechanisms illustrated as backplane 436 (e.g., a first fullmesh coupling the line cards and a second full mesh coupling all of thecards).

Returning to FIG. 4A, the general purpose network device 404 includeshardware 440 comprising a set of one or more processor(s) 442 (which areoften COTS processors) and network interface controller(s) 444 (NICs;also known as network interface cards) (which include physical NIs 446),as well as non-transitory machine readable storage media 448 havingstored therein software 450. During operation, the processor(s) 442execute the software 450 to instantiate a hypervisor 454 (sometimesreferred to as a virtual machine monitor (VMM)) and one or more virtualmachines 462A-R that are run by the hypervisor 454, which arecollectively referred to as software instance(s) 452. A virtual machineis a software implementation of a physical machine that runs programs asif they were executing on a physical, non-virtualized machine; andapplications generally do not know they are running on a virtual machineas opposed to running on a “bare metal” host electronic device, thoughsome systems provide para-virtualization which allows an operatingsystem or application to be aware of the presence of virtualization foroptimization purposes. Each of the virtual machines 462A-R, and thatpart of the hardware 440 that executes that virtual machine (be ithardware dedicated to that virtual machine and/or time slices ofhardware temporally shared by that virtual machine with others of thevirtual machine(s) 462A-R), forms a separate virtual network element(s)460A-R. In some embodiments, the virtual machine module 462A encompassesa BFD module 463A that manages the configuration of BFD session betweenthe network device and another network device. The return path module491B can similarly facilitate the establishment of BFD sessions, inparticular, where it is desirable to have separate forward and returnpaths for the BFD sessions.

The virtual network element(s) 460A-R perform similar functionality tothe virtual network element(s) 430A-R. For instance, the hypervisor 454may present a virtual operating platform that appears like networkinghardware 410 to virtual machine 462A, and the virtual machine 462A maybe used to implement functionality similar to the control communicationand configuration module(s) 432A and forwarding table(s) 434A (thisvirtualization of the hardware 440 is sometimes referred to as networkfunction virtualization (NFV)). Thus, NFV may be used to consolidatemany network equipment types onto industry standard high volume serverhardware, physical switches, and physical storage, which could belocated in Data centers, NDs, and customer premise equipment (CPE).However, different embodiments of the invention may implement one ormore of the virtual machine(s) 462A-R differently. For example, whileembodiments of the invention are illustrated with each virtual machine462A-R corresponding to one VNE 460A-R, alternative embodiments mayimplement this correspondence at a finer level granularity (e.g., linecard virtual machines virtualize line cards, control card virtualmachine virtualize control cards, etc.); it should be understood thatthe techniques described herein with reference to a correspondence ofvirtual machines to VNEs also apply to embodiments where such a finerlevel of granularity is used.

In certain embodiments, the hypervisor 454 includes a virtual switchthat provides similar forwarding services as a physical Ethernet switch.Specifically, this virtual switch forwards traffic between virtualmachines and the NIC(s) 444, as well as optionally between the virtualmachines 462A-R; in addition, this virtual switch may enforce networkisolation between the VNEs 460A-R that by policy are not permitted tocommunicate with each other (e.g., by honoring virtual local areanetworks (VLANs)).

The third exemplary ND implementation in FIG. 4A is a hybrid networkdevice 06, which includes both custom ASICs/proprietary OS and COTSprocessors/standard OS in a single ND or a single card within an ND. Incertain embodiments of such a hybrid network device, a platform VM(i.e., a VM that that implements the functionality of thespecial-purpose network device 402) could provide forpara-virtualization to the networking hardware present in the hybridnetwork device 406.

Regardless of the above exemplary implementations of an ND, when asingle one of multiple VNEs implemented by an ND is being considered(e.g., only one of the VNEs is part of a given virtual network) or whereonly a single VNE is currently being implemented by an ND, the shortenedterm network element (NE) is sometimes used to refer to that VNE. Alsoin all of the above exemplary implementations, each of the VNEs (e.g.,VNE(s) 430A-R, VNEs 460A-R, and those in the hybrid network device 406)receives data on the physical NIs (e.g., 416, 446) and forwards thatdata out the appropriate ones of the physical NIs (e.g., 416, 446). Forexample, a VNE implementing IP router functionality forwards IP packetson the basis of some of the IP header information in the IP packet;where IP header information includes source IP address, destination IPaddress, source port, destination port (where “source port” and“destination port” refer herein to protocol ports, as opposed tophysical ports of a ND), transport protocol (e.g., user datagramprotocol (UDP) (RFC 768, 2460, 2675, 4113, and 5405), TransmissionControl Protocol (TCP) (RFC 793 and 1180), and differentiated services(DSCP) values (RFC 2474, 2475, 2597, 2983, 3086, 3140, 3246, 3247, 3260,4594, 5865, 3289, 3290, and 3317).

FIG. 4C illustrates various exemplary ways in which VNEs may be coupledaccording to some embodiments of the invention. FIG. 4C shows VNEs470A.1-470A.P (and optionally VNEs 470A.Q-470A.R) implemented in ND 400Aand VNE 470H.1 in ND 400H. In FIG. 4C, VNEs 470A.1-P are separate fromeach other in the sense that they can receive packets from outside ND400A and forward packets outside of ND 400A; VNE 470A.1 is coupled withVNE 470H.1, and thus they communicate packets between their respectiveNDs; VNE 470A.2-470A.3 may optionally forward packets between themselveswithout forwarding them outside of the ND 400A; and VNE 470A.P mayoptionally be the first in a chain of VNEs that includes VNE 470A.Qfollowed by VNE 470A.R (this is sometimes referred to as dynamic servicechaining, where each of the VNEs in the series of VNEs provides adifferent service—e.g., one or more layer 4-7 network services). WhileFIG. 4C illustrates various exemplary relationships between the VNEs,alternative embodiments may support other relationships (e.g.,more/fewer VNEs, more/fewer dynamic service chains, multiple differentdynamic service chains with some common VNEs and some different VNEs).

The NDs of FIG. 4A, for example, may form part of the Internet or aprivate network; and other electronic devices (not shown; such as enduser devices including workstations, laptops, netbooks, tablets, palmtops, mobile phones, smartphones, multimedia phones, Voice Over InternetProtocol (VOIP) phones, terminals, portable media players, GPS units,wearable devices, gaming systems, set-top boxes, Internet enabledhousehold appliances) may be coupled to the network (directly or throughother networks such as access networks) to communicate over the network(e.g., the Internet or virtual private networks (VPNs) overlaid on(e.g., tunneled through) the Internet) with each other (directly orthrough servers) and/or access content and/or services. Such contentand/or services are typically provided by one or more servers (notshown) belonging to a service/content provider or one or more end userdevices (not shown) participating in a peer-to-peer (P2P) service, andmay include, for example, public webpages (e.g., free content, storefronts, search services), private webpages (e.g., username/passwordaccessed webpages providing email services), and/or corporate networksover VPNs. For instance, end user devices may be coupled (e.g., throughcustomer premise equipment coupled to an access network (wired orwirelessly)) to edge NDs, which are coupled (e.g., through one or morecore NDs) to other edge NDs, which are coupled to electronic devicesacting as servers. However, through compute and storage virtualization,one or more of the electronic devices operating as the NDs in FIG. 4Amay also host one or more such servers (e.g., in the case of the generalpurpose network device 404, one or more of the virtual machines 462A-Rmay operate as servers; the same would be true for the hybrid networkdevice 406; in the case of the special-purpose network device 402, oneor more such servers could also be run on a hypervisor executed by thecompute resource(s) 412); in which case the servers are said to beco-located with the VNEs of that ND.

A virtual network is a logical abstraction of a physical network (suchas that in FIG. 4A) that provides network services (e.g., L2 and/or L3services). A virtual network can be implemented as an overlay network(sometimes referred to as a network virtualization overlay) thatprovides network services (e.g., layer 2 (L2, data link layer) and/orlayer 3 (L3, network layer) services) over an underlay network (e.g., anL3 network, such as an Internet Protocol (IP) network that uses tunnels(e.g., generic routing encapsulation (GRE), layer 2 tunneling protocol(L2TP), IPSec) to create the overlay network).

A network virtualization edge (NVE) sits at the edge of the underlaynetwork and participates in implementing the network virtualization; thenetwork-facing side of the NVE uses the underlay network to tunnelframes to and from other NVEs; the outward-facing side of the NVE sendsand receives data to and from systems outside the network. A virtualnetwork instance (VNI) is a specific instance of a virtual network on aNVE (e.g., a NE/VNE on an ND, a part of a NE/VNE on a ND where thatNE/VNE is divided into multiple VNEs through emulation); one or moreVNIs can be instantiated on an NVE (e.g., as different VNEs on an ND). Avirtual access point (VAP) is a logical connection point on the NVE forconnecting external systems to a virtual network; a VAP can be physicalor virtual ports identified through logical interface identifiers (e.g.,a VLAN ID).

Examples of network services include: 1) an Ethernet LAN emulationservice (an Ethernet-based multipoint service similar to an InternetEngineering Task Force (IETF) Multiprotocol Label Switching (MPLS) orEthernet VPN (EVPN) service) in which external systems areinterconnected across the network by a LAN environment over the underlaynetwork (e.g., an NVE provides separate L2 VNIs (virtual switchinginstances) for different such virtual networks, and L3 (e.g., IP/MPLS)tunneling encapsulation across the underlay network); and 2) avirtualized IP forwarding service (similar to IETF IP VPN (e.g., BorderGateway Protocol (BGP)/MPLS IPVPN RFC 4364) from a service definitionperspective) in which external systems are interconnected across thenetwork by an L3 environment over the underlay network (e.g., an NVEprovides separate L3 VNIs (forwarding and routing instances) fordifferent such virtual networks, and L3 (e.g., IP/MPLS) tunnelingencapsulation across the underlay network)). Network services may alsoinclude quality of service capabilities (e.g., traffic classificationmarking, traffic conditioning and scheduling), security capabilities(e.g., filters to protect customer premises from network—originatedattacks, to avoid malformed route announcements), and managementcapabilities (e.g., full detection and processing).

FIG. 4D illustrates a network with a single network element on each ofthe NDs of FIG. 4A, and within this straight forward approach contrastsa traditional distributed approach (commonly used by traditionalrouters) with a centralized approach for maintaining reachability andforwarding information (also called network control), according to someembodiments of the invention. Specifically, FIG. 4D illustrates networkelements (NEs) 470A-H with the same connectivity as the NDs 400A-H ofFIG. 4A.

FIG. 4D illustrates that the distributed approach 472 distributesresponsibility for generating the reachability and forwardinginformation across the NEs 470A-H; in other words, the process ofneighbor discovery and topology discovery is distributed.

For example, where the special-purpose network device 402 is used, thecontrol communication and configuration module(s) 432A-R of the NDcontrol plane 424 typically include a reachability and forwardinginformation module to implement one or more routing protocols (e.g., anexterior gateway protocol such as Border Gateway Protocol (BGP) (RFC4271), Interior Gateway Protocol(s) (IGP) (e.g., Open Shortest PathFirst (OSPF) (RFC 2328 and 5340), Intermediate System to IntermediateSystem (IS-IS) (RFC 1142), Routing Information Protocol (RIP) (version 1RFC 1058, version 2 RFC 2453, and next generation RFC 2080)), LabelDistribution Protocol (LDP) (RFC 5036), Resource Reservation Protocol(RSVP) (RFC 2205, 2210, 2211, 2212, as well as RSVP-Traffic Engineering(TE): Extensions to RSVP for LSP Tunnels RFC 3209, GeneralizedMulti-Protocol Label Switching (GMPLS) Signaling RSVP-TE RFC 3473, RFC3936, 4495, and 4558)) that communicate with other NEs to exchangeroutes, and then selects those routes based on one or more routingmetrics. Thus, the NEs 470A-H (e.g., the compute resource(s) 412executing the control communication and configuration module(s) 432A-R)perform their responsibility for participating in controlling how data(e.g., packets) is to be routed (e.g., the next hop for the data and theoutgoing physical NI for that data) by distributively determining thereachability within the network and calculating their respectiveforwarding information. Routes and adjacencies are stored in one or morerouting structures (e.g., Routing Information Base (RIB), LabelInformation Base (LIB), one or more adjacency structures) on the NDcontrol plane 424. The ND control plane 424 programs the ND forwardingplane 426 with information (e.g., adjacency and route information) basedon the routing structure(s). For example, the ND control plane 424programs the adjacency and route information into one or more forwardingtable(s) 434A-R (e.g., Forwarding Information Base (FIB), LabelForwarding Information Base (LFIB), and one or more adjacencystructures) on the ND forwarding plane 426. For layer 2 forwarding, theND can store one or more bridging tables that are used to forward databased on the layer 2 information in that data. While the above exampleuses the special-purpose network device 402, the same distributedapproach 472 can be implemented on the general purpose network device404 and the hybrid network device 406.

FIG. 4D illustrates that a centralized approach 474 (also known assoftware defined networking (SDN)) that decouples the system that makesdecisions about where traffic is sent from the underlying systems thatforwards traffic to the selected destination. The illustratedcentralized approach 474 has the responsibility for the generation ofreachability and forwarding information in a centralized control plane476 (sometimes referred to as a SDN control module, controller, networkcontroller, OpenFlow controller, SDN controller, control plane node,network virtualization authority, or management control entity), andthus the process of neighbor discovery and topology discovery iscentralized. The centralized control plane 476 has a south boundinterface 482 with a data plane 480 (sometime referred to theinfrastructure layer, network forwarding plane, or forwarding plane(which should not be confused with a ND forwarding plane)) that includesthe NEs 470A-H (sometimes referred to as switches, forwarding elements,data plane elements, or nodes). The centralized control plane 476includes a network controller 478, which includes a centralizedreachability and forwarding information module 479 that determines thereachability within the network and distributes the forwardinginformation to the NEs 470A-H of the data plane 480 over the south boundinterface 482 (which may use the OpenFlow protocol). Thus, the networkintelligence is centralized in the centralized control plane 476executing on electronic devices that are typically separate from theNDs.

For example, where the special-purpose network device 402 is used in thedata plane 480, each of the control communication and configurationmodule(s) 432A-R of the ND control plane 424 typically include a controlagent that provides the VNE side of the south bound interface 482. Inthis case, the ND control plane 424 (the compute resource(s) 412executing the control communication and configuration module(s) 432A-R)performs its responsibility for participating in controlling how data(e.g., packets) is to be routed (e.g., the next hop for the data and theoutgoing physical NI for that data) through the control agentcommunicating with the centralized control plane 476 to receive theforwarding information (and in some cases, the reachability information)from the centralized reachability and forwarding information module 479(it should be understood that in some embodiments of the invention, thecontrol communication and configuration module(s) 432A-R, in addition tocommunicating with the centralized control plane 476, may also play somerole in determining reachability and/or calculating forwardinginformation—albeit less so than in the case of a distributed approach;such embodiments are generally considered to fall under the centralizedapproach 474, but may also be considered a hybrid approach). In someembodiments, the centralized reachability and forwarding informationmodule 479 encompasses BFD functions in a BFD module 481 that managesthe establishment of BFD sessions between the network devices. A returnpath module 491 can similarly manage the establishment of explicitreturn paths between these network devices. In this embodiment, the BFDsession does not need to be bootstrapped using the LSP ping, thus, theBFD module 481 and return path module 491 have visibility to identifythe forward path and return path and to configure each of the end nodesto send the BFD control packets along the appropriate path (i.e., theforward path or return path). Similarly, the return path module 491 canverify the availability of these return paths.

While the above example uses the special-purpose network device 402, thesame centralized approach 474 can be implemented with the generalpurpose network device 404 (e.g., each of the VNE 460A-R performs itsresponsibility for controlling how data (e.g., packets) is to be routed(e.g., the next hop for the data and the outgoing physical NI for thatdata) by communicating with the centralized control plane 476 to receivethe forwarding information (and in some cases, the reachabilityinformation) from the centralized reachability and forwardinginformation module 479; it should be understood that in some embodimentsof the invention, the VNEs 460A-R, in addition to communicating with thecentralized control plane 476, may also play some role in determiningreachability and/or calculating forwarding information—albeit less sothan in the case of a distributed approach) and the hybrid networkdevice 406. In fact, the use of SDN techniques can enhance the NFVtechniques typically used in the general purpose network device 404 orhybrid network device 406 implementations as NFV is able to support SDNby providing an infrastructure upon which the SDN software can be run,and NFV and SDN both aim to make use of commodity server hardware andphysical switches.

FIG. 4D also shows that the centralized control plane 476 has a northbound interface 484 to an application layer 486, in which residesapplication(s) 488. The centralized control plane 476 has the ability toform virtual networks 492 (sometimes referred to as a logical forwardingplane, network services, or overlay networks (with the NEs 470A-H of thedata plane 480 being the underlay network)) for the application(s) 488.Thus, the centralized control plane 476 maintains a global view of allNDs and configured NEs/VNEs, and it maps the virtual networks to theunderlying NDs efficiently (including maintaining these mappings as thephysical network changes either through hardware (ND, link, or NDcomponent) failure, addition, or removal). In some embodiments, the PCE499 as described herein above and/or an associated PCA (not shown) canbe implemented at the application layer 486.

While FIG. 4D shows the distributed approach 472 separate from thecentralized approach 474, the effort of network control may bedistributed differently or the two combined in certain embodiments ofthe invention. For example: 1) embodiments may generally use thecentralized approach (SDN) 474, but have certain functions delegated tothe NEs (e.g., the distributed approach may be used to implement one ormore of fault monitoring, performance monitoring, protection switching,and primitives for neighbor and/or topology discovery); or 2)embodiments of the invention may perform neighbor discovery and topologydiscovery via both the centralized control plane and the distributedprotocols, and the results compared to raise exceptions where they donot agree. Such embodiments are generally considered to fall under thecentralized approach 474, but may also be considered a hybrid approach.

While FIG. 4D illustrates the simple case where each of the NDs 400A-Himplements a single NE 470A-H, it should be understood that the networkcontrol approaches described with reference to FIG. 4D also work fornetworks where one or more of the NDs 400A-H implement multiple VNEs(e.g., VNEs 430A-R, VNEs 460A-R, those in the hybrid network device406). Alternatively or in addition, the network controller 478 may alsoemulate the implementation of multiple VNEs in a single ND.Specifically, instead of (or in addition to) implementing multiple VNEsin a single ND, the network controller 478 may present theimplementation of a VNE/NE in a single ND as multiple VNEs in thevirtual networks 492 (all in the same one of the virtual network(s) 492,each in different ones of the virtual network(s) 492, or somecombination). For example, the network controller 478 may cause an ND toimplement a single VNE (a NE) in the underlay network, and thenlogically divide up the resources of that NE within the centralizedcontrol plane 476 to present different VNEs in the virtual network(s)492 (where these different VNEs in the overlay networks are sharing theresources of the single VNE/NE implementation on the ND in the underlaynetwork).

On the other hand, FIGS. 4E and 4F respectively illustrate exemplaryabstractions of NEs and VNEs that the network controller 478 may presentas part of different ones of the virtual networks 492. FIG. 4Eillustrates the simple case of where each of the NDs 400A-H implements asingle NE 470A-H (see FIG. 4D), but the centralized control plane 476has abstracted multiple of the NEs in different NDs (the NEs 470A-C andG-H) into (to represent) a single NE 470I in one of the virtualnetwork(s) 492 of FIG. 4D, according to some embodiments of theinvention. FIG. 4E shows that in this virtual network, the NE 470I iscoupled to NE 470D and 470F, which are both still coupled to NE 470E.

FIG. 4F illustrates a case where multiple VNEs (VNE 470A.1 and VNE470H.1) are implemented on different NDs (ND 400A and ND 400H) and arecoupled to each other, and where the centralized control plane 476 hasabstracted these multiple VNEs such that they appear as a single VNE470T within one of the virtual networks 492 of FIG. 4D, according tosome embodiments of the invention. Thus, the abstraction of a NE or VNEcan span multiple NDs.

While some embodiments of the invention implement the centralizedcontrol plane 476 as a single entity (e.g., a single instance ofsoftware running on a single electronic device), alternative embodimentsmay spread the functionality across multiple entities for redundancyand/or scalability purposes (e.g., multiple instances of softwarerunning on different electronic devices).

Similar to the network device implementations, the electronic device(s)running the centralized control plane 476, and thus the networkcontroller 478 including the centralized reachability and forwardinginformation module 479, may be implemented a variety of ways (e.g., aspecial purpose device, a general-purpose (e.g., COTS) device, or hybriddevice). These electronic device(s) would similarly include computeresource(s), a set or one or more physical NICs, and a non-transitorymachine-readable storage medium having stored thereon the centralizedcontrol plane software. For instance, FIG. 5 illustrates, a generalpurpose control plane device 504 including hardware 540 comprising a setof one or more processor(s) 542 (which are often COTS processors) andnetwork interface controller(s) 544 (NICs; also known as networkinterface cards) (which include physical NIs 546), as well asnon-transitory machine readable storage media 548 having stored thereincentralized control plane (CCP) software 550.

In embodiments that use compute virtualization, the processor(s) 542typically execute software to instantiate a hypervisor 554 (sometimesreferred to as a virtual machine monitor (VMM)) and one or more virtualmachines 562A-R that are run by the hypervisor 554; which arecollectively referred to as software instance(s) 552. A virtual machineis a software implementation of a physical machine that runs programs asif they were executing on a physical, non-virtualized machine; andapplications generally are not aware they are running on a virtualmachine as opposed to running on a “bare metal” host electronic device,though some systems provide para-virtualization which allows anoperating system or application to be aware of the presence ofvirtualization for optimization purposes. Again, in embodiments wherecompute virtualization is used, during operation an instance of the CCPsoftware 550 (illustrated as CCP instance 576A) on top of an operatingsystem 564A are typically executed within the virtual machine 562A. Inembodiments where compute virtualization is not used, the CCP instance1576A on top of operating system 564A is executed on the “bare metal”general purpose control plane device 504.

The operating system 564A provides basic processing, input/output (I/O),and networking capabilities. In some embodiments, the CCP instance 576Aincludes a network controller instance 578. The network controllerinstance 578 includes a centralized reachability and forwardinginformation module instance 579 (which is a middleware layer providingthe context of the network controller 578 to the operating system 564Aand communicating with the various NEs), and an CCP application layer580 (sometimes referred to as an application layer) over the middlewarelayer (providing the intelligence required for various networkoperations such as protocols, network situational awareness, anduser—interfaces). At a more abstract level, this CCP application layer580 within the centralized control plane 476 works with virtual networkview(s) (logical view(s) of the network), and the middleware layerprovides the conversion from the virtual networks to the physical view.The CCP application layer 580 can encompass the functionality of the BFDmodule 581 and the return path module 591 as described herein above.

The centralized control plane 476 transmits relevant messages to thedata plane 480 based on CCP application layer 580 calculations andmiddleware layer mapping for each flow. A flow may be defined as a setof packets whose headers match a given pattern of bits; in this sense,traditional IP forwarding is also flow-based forwarding where the flowsare defined by the destination IP address for example; however, in otherimplementations, the given pattern of bits used for a flow definitionmay include more fields (e.g., 10 or more) in the packet headers.Different NDs/NEs/VNEs of the data plane 480 may receive differentmessages, and thus different forwarding information. The data plane 480processes these messages and programs the appropriate flow informationand corresponding actions in the forwarding tables (sometime referred toas flow tables) of the appropriate NE/VNEs, and then the NEs/VNEs mapincoming packets to flows represented in the forwarding tables andforward packets based on the matches in the forwarding tables.

Standards such as OpenFlow define the protocols used for the messages,as well as a model for processing the packets. The model for processingpackets includes header parsing, packet classification, and makingforwarding decisions. Header parsing describes how to interpret a packetbased upon a well-known set of protocols. Some protocol fields are usedto build a match structure (or key) that will be used in packetclassification (e.g., a first key field could be a source media accesscontrol (MAC) address, and a second key field could be a destination MACaddress).

Packet classification involves executing a lookup in memory to classifythe packet by determining which entry (also referred to as a forwardingtable entry or flow entry) in the forwarding tables best matches thepacket based upon the match structure, or key, of the forwarding tableentries. It is possible that many flows represented in the forwardingtable entries can correspond/match to a packet; in this case the systemis typically configured to determine one forwarding table entry from themany according to a defined scheme (e.g., selecting a first forwardingtable entry that is matched). Forwarding table entries include both aspecific set of match criteria (a set of values or wildcards, or anindication of what portions of a packet should be compared to aparticular value/values/wildcards, as defined by the matchingcapabilities—for specific fields in the packet header, or for some otherpacket content), and a set of one or more actions for the data plane totake on receiving a matching packet. For example, an action may be topush a header onto the packet, for the packet using a particular port,flood the packet, or simply drop the packet. Thus, a forwarding tableentry for IPv4/IPv6 packets with a particular transmission controlprotocol (TCP) destination port could contain an action specifying thatthese packets should be dropped.

Making forwarding decisions and performing actions occurs, based uponthe forwarding table entry identified during packet classification, byexecuting the set of actions identified in the matched forwarding tableentry on the packet.

However, when an unknown packet (for example, a “missed packet” or a“match-miss” as used in OpenFlow parlance) arrives at the data plane480, the packet (or a subset of the packet header and content) istypically forwarded to the centralized control plane 476. Thecentralized control plane 476 will then program forwarding table entriesinto the data plane 480 to accommodate packets belonging to the flow ofthe unknown packet. Once a specific forwarding table entry has beenprogrammed into the data plane 480 by the centralized control plane 476,the next packet with matching credentials will match that forwardingtable entry and take the set of actions associated with that matchedentry.

For example, while the flow diagrams in the figures show a particularorder of operations performed by certain embodiments of the invention,it should be understood that such order is exemplary (e.g., alternativeembodiments may perform the operations in a different order, combinecertain operations, overlap certain operations, etc.).

Those skilled in the art will appreciate that the use of the term“exemplary” is used herein to mean “illustrative,” or “serving as anexample,” and is not intended to imply that a particular embodiment ispreferred over another or that a particular feature is essential.Likewise, the terms “first” and “second,” and similar terms, are usedsimply to distinguish one particular instance of an item or feature fromanother, and do not indicate a particular order or arrangement, unlessthe context clearly indicates otherwise. Further, the term “step,” asused herein, is meant to be synonymous with “operation” or “action.” Anydescription herein of a sequence of steps does not imply that theseoperations must be carried out in a particular order, or even that theseoperations are carried out in any order at all, unless the context orthe details of the described operation clearly indicates otherwise.

Of course, the present invention may be carried out in other specificways than those herein set forth without departing from the scope andessential characteristics of the invention. One or more of the specificprocesses discussed above may be carried out using one or moreappropriately configured processing circuits. In some embodiments, theseprocessing circuits may comprise one or more microprocessors,microcontrollers, and/or digital signal processors programmed withappropriate software and/or firmware to carry out one or more of theoperations described above, or variants thereof. In some embodiments,these processing circuits may comprise customized hardware to carry outone or more of the functions described above. The present embodimentsare, therefore, to be considered in all respects as illustrative and notrestrictive.

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described, can be practiced with modificationand alteration within the spirit and scope of the appended claims. Thedescription is thus to be regarded as illustrative instead of limiting.

1. A method to establish a bidirectional forwarding detection, BFD,session with an explicit return path to enable detection of data planefailures between an active node and a passive node in a network in theexplicit return path, the method for the passive node comprising:receiving a label switched path, LSP, ping including a BFD discriminatortype length value, TLV, of the active node and a BFD return path TLVincluding information of a return path to the active node; verifyingavailability of the return path; and sending BFD control packets to theactive node using the return path to detect a failure on the returnpath, in response to verified availability of the return path.
 2. Themethod of claim 1, further comprising: associating the return path withthe BFD discriminator TLV of the active node.
 3. The method of claim 1,further comprising: notifying the active node of an error using a LSPping reply in response to the verifying indicating that the return pathis not available or valid.
 4. The method of claim 1, wherein theverifying includes verifying a Target Forwarding Equivalency Class, FEC,Stack TLV in the LSP ping.
 5. The method of claim 1, wherein the returnpath is indicated by a multi-protocol label, MPLS, stack.
 6. A networkdevice functioning as a passive node, the network device configured toestablish a bidirectional forwarding detection, BFD, session with anexplicit return path to enable detection of data plane failures betweenan active node and the passive node in the explicit return path, thenetwork device comprising: a non-transitory machine-readable storagemedium configured to store a BFD module and a return path module; and anetwork processor communicatively coupled to the non-transitorymachine-readable storage medium, the network processor configured toexecute the BFD module and the return path module, the BFD moduleconfigured to receive a label switched path, LSP, ping including a BFDdiscriminator type length value, TLV, of the active node and a BFDreturn path TLV including information of a return path to the activenode, the return path module configured to verify availability of thereturn path; and send BFD control packets to the active node using thereturn path to detect a failure on the return path, in response toverified availability of the return path.
 7. The network device of claim6, wherein the return path module is further to associate the returnpath with the BFD discriminator TLV of the active node.
 8. The networkdevice of claim 6, wherein the return path module is further to notifythe active node of an error using a LSP ping reply in response to theverifying indicating that the return path is not available or valid. 9.The network device of claim 6, wherein the return path module verifies aTarget Forwarding Equivalency Class, FEC, Stack TLV in the LSP ping. 10.The network device of claim 6, wherein the return path is indicated by amulti-protocol label, MPLS, stack.
 11. A computing device executing aplurality of virtual machines for implementing network functionvirtualization, NFV, wherein a virtual machine from the plurality ofvirtual machines is configured to establish a bidirectional forwardingdetection, BFD, session with an explicit return path to enable detectionof data plane failures between an active node and a passive node in theexplicit return path, the virtual machine instantiated by a hypervisoror virtual machine monitor, the virtual machine functioning as thepassive node, the computing device comprising: a non-transitorymachine-readable storage medium configured to store a BFD module and areturn path module; and a processor communicatively coupled to thenon-transitory machine-readable storage medium, the processor configuredto execute the virtual machine that executes the BFD module and thereturn path module, the BFD module configured to receive a labelswitched path, LSP, ping including a BFD discriminator type lengthvalue, TLV, of the active node and a BFD return path TLV includinginformation of a return path to the active node, the return path moduleconfigured to verify availability of the return path; and send BFDcontrol packets to the active node using the return path to detect afailure on the return path, in response to verified availability of thereturn path.
 12. The computing device of claim 11, wherein the returnpath module is further to associate the return path with the BFDdiscriminator TLV of the active node.
 13. The computing device of claim11, wherein the return path module is further to notify the active nodeof an error using a LSP ping reply in response to the verifyingindicating that the return path is not available or valid.
 14. Thecomputing device of claim 11, wherein the return path module verifies aTarget Forwarding Equivalency Class, FEC, Stack TLV in the LSP ping. 15.The computing device of claim 11, wherein the return path is indicatedby a multi-protocol label, MPLS, stack.
 16. A control plane deviceconfigured to implement at least one centralized control plane for asoftware defined network, SDN, the centralized control plane configuredto establish a bidirectional forwarding detection, BFD, session with anexplicit return path to enable detection of data plane failures betweenan active node and a passive node in the explicit return path, thecontrol plane device servicing the passive node, the control planedevice comprising: a non-transitory machine-readable storage mediumconfigured to store a BFD module and a return path module; and aprocessor communicatively coupled to the non-transitory machine-readablestorage medium, the processor configured to execute the BFD module andthe return path module, the BFD module configured to receive a labelswitched path, LSP, ping including a BFD discriminator type lengthvalue, TLV, of the active node and a BFD return path TLV includinginformation of a return path to the active node, the return path moduleconfigured to verify availability of the return path; and send BFDcontrol packets to the active node using the return path to detect afailure on the return path, in response to verified availability of thereturn path.
 17. The control plane device of claim 16, wherein thereturn path module is further to associate the return path with the BFDdiscriminator TLV of the active node.
 18. The control plane device ofclaim 16, wherein the return path module is further to notify the activenode of an error using a LSP ping reply in response to the verifyingindicating that the return path is not available or valid.
 19. Thecontrol plane device of claim 16, wherein the return path moduleverifies a Target Forwarding Equivalency Class, FEC, Stack TLV in theLSP ping.
 20. The control plane device of claim 16, wherein the returnpath is indicated by a multi-protocol label, MPLS, stack.